9.1.3.3 UID Check

The next step is to consider the entry's UID. UIDs are used for a variety
of purposes and must be part of all user entries and may be included in group
entries. A user entry without a UID is ignored and processing of this alias
URL terminates unsuccessfully. UIDs for entries in a hosted domain can consist
of the real UID, a separator character, and then a domain. The MTA only wants
the real UID so the rest is removed if present using the domain separator
character obtained with the LDAP_DOMAIN_ATTR_UID_SEPARATOR MTA
option in the option.dat file.

In the unlikely event that an attribute other than uid is
used to store UIDs, the LDAP_UID MTA option can be used
to force use of a different attribute.